Chromian Diopside - The Emerald of Diamond Deposits

Chromian Diopside, also known as the Cape Emerald, produce gemstones that rival emerald. These gems are common in some kimberlite & lamprophyre pipes, but are often disgarded in the search for diamonds.

Monday, February 8, 2010

Chromian diopside (emerald green) with pyrope and almandine garnet collected from anthills in the Green River Basin of Wyoming. One pyrope(center), was faceted.

Chromian diopside - gemstones from the Earth's Mantle - Chromian diopside is a very rare emerald-green mineral with hardness similar to opal: it does not quite match the hardness of emerald which it mimics in faceted stones. Even though it is found in many places in North America and elsewhere in the world, it is extremely rare because of being found almost exclusively in rare mafic and ultramafic igneous and metamorphic rocks (dark colored rocks with high chromium, magnesium and potassium and low silica).
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Artist's cartoon of a kimberlite
pipe in cross section. When such
volcanoes contain diamond, they
can be worth $billions of dollars.
More than 100 such pipes and dikes
have been found in Colorado &
Wyoming with diamonds and
chromian diopside.

One of the principal host rocks is kimberlite. The GemHunter found chromian diopside in many kimberlites in the US, in several lamprophyres in Montana and Wyoming, in anthills and in serpentinites in northern California. Kimberlite is considered to be one of the rarest rock types on the surface of the earth by volume – possibly only lamproite and lamprophyre are rarer. When found, kimberlite occurs in small, carrot-shaped pipes (maar volcanoes) with small surface expressions that are generally less than 3,000 feet across (most are smaller ranging from narrow dikes to elongate pipes a couple of hundred feet across). In these pipes, chromian diopside is usually found as disseminated mineral grains, megacrysts (large crystals) and in rare mafic and ultramafic nodules and xenoliths. The chromian diopside is typically more abundant than diamond and because of its distinctive color, it is readily visible. However, the value of the stone is not high enough to mine kimberlite for chromian diopside, and essentially all diamond mines ignore the mineral and only mine for diamond.

These eruptions have been liken to a shotgun blast shot from the earth. Many xenolith rocks in these volcanoes are shot from the mantle originating at depths of 90 to 120 miles. When erupted, most of these rocks are rounded. Others are plucked from the wall rocks at the surface all the way down to the mantle. Angular blocks and boulders are down-dropped in the magma from the surface in the rapidly cooling magma. At the Sloan kimberlite in Colorado, the GemHunter was able to examine blocks of rock that were the size of a golf cart in the rib of the Sloan 2 underground mine. These are common in many kimberlite pipes.

The maar volcano at the Maxwell kimberlite (Colorado) shows a distinct shallow depression with different vegetation than the surrounding granitic country rock.

Now imagine standing near this volcano. Hold on to your hat! The magma and gases are under incredible pressure and shoot up from the mantle to the surface in a few hours at most. The eruption does not produce a volcanic cone, instead it forms a maar that looks like an impact crater. For years, the Winkler kimberlite in Kansas was interpreted as an impact, but within the past few decades, it was discovered to be a kimberlite maar.

If you were standing near a kimberlite eruption, you would hear a sonic boom! The gas from this magma being under incredible pressure erupts at Mach 2 to Mach 3. Remember those jets in years past when they broke the sound barrier – they were many miles away! Not only might this destroy your ear drums, you would likely suffocate because of all of the carbon dioxide pushing away oxygen - if you were a tree, you would likely enjoy the added CO2. Even though CO2 is harmless (in spite of the EPA) in normal concentrations, such massive amounts would push all oxygen out of the immediate area leaving you with no air to breath. Now if somehow this didn’t kill you, you would have to avoid cannon balls of mantle nodules and diamonds shot from the volcano – these would be like BBs from a shotgun blast.

As the CO2 expands it will freeze everything in the area! Yep, instead of burning, you would likely catch a cold (if you are still alive). It has been estimated that the magma temperature at eruption would cool to 32oF. This is why there is usually no evidence of baking of the country rock next to kimberlite magma.

In the Colorado-Montana-Wyoming region, there are many kimberlites, lamproites and lamprophyres. Nearly all that have been tested contain diamond. Wouldn’t you think that the States would investigate these resources. If they would have, they might not be facing bankruptcy. Some diamonds have sold for more than 200,000 times an equivalent weight in gold! Imagine what a few dozen diamond mines would do for the economy and tax base.

In the past, research for these kinds of projects at the Wyoming Geological Survey were poorly funded and often measured in the level of a few hundred dollars/year to search for gemstones, precious and base metals as well as conduct regional geological mapping projects, lectures, write publications, assist prospectors and companies, etc. Yet with this poverty level of funding we found evidence of a major diamond province. In Canada, each kimberlite discovery is estimated to cost a minimum of $1.5 million. In Colorado and Wyoming, more than 100 kimberlites were discovered on a budget of about $30,000 over 30 years! Now imagine what $1.5 million would have done! One could write a book about this!

In this aerial photo, large cryptovolcanic structures occur in a field of >50 probable kimberlitepipes I discovered a few years ago. Several million people drive by these every year not paying realizing they are driving next to some probable diamond mines along I-80! The white color in the depressions is calcium carbonate. Due to CO2 gas, much of it becomes fixed as calcium carbonate upon cooling. Country rock surrounding these depressions is dominated by silicate minerals with no obvious source of calcium carbonate. Over the past few years, nine similar districts that include a few hundred similar cryptovolcanic structures – nearly all are unexplored were found.

Chromian diopside, a chrome-rich variety of the mineral diopside [CaMg(SiO3)2], typically has an exceptional emerald-green color due to the substitution of chrome (1 to 2.8%) for magnesium in its crystal lattice. Generally, the greater amount of chrome in the crystal lattice, the more intense will be the emerald-green color. The mineral has a hardness of 6 to 6.5, specific gravity of 3.2 to 3.4, and forms monoclinic crystals. In other words, well-formed diopside crystals give the appearance of a box that is compressed in one direction.

The gemstone is referred to by its mineralogical name (chromian diopside), but it has also been referred to as chrome diopside, chromian pyroxene, and as Cape Emerald, a misnomer after the first cut specimens from Cape Town, South Africa. Since the term Cape Emerald was first applied to gem-quality prehnite, to avoid confusion, it would be more appropriate to refer to chromian diopside as ‘Northern Cape Emerald’ based on its type locality at Kimberley in the Northern Cape Province of South Africa, where the gem was recovered from diamondiferous kimberlite along with Cape Ruby (pyrope garnet) (see the November, 2009 GemHunter newsletter) and of course, diamond.

Emerald green chromian diopside surrounded by pyrope garnets. This
material was collected from anthills in Butcherknife Draw south of Green
River, Wyoming and later faceted in Sri Lanka.

Chromian enstatite, a chrome-rich variety of enstatite (MgSiO2), has a hardness of 5.5 to 6 and specific gravity of 3.1 to 3.3, and occurs as orthorhombic crystals with similar emerald-green color as chromian diopside. Both chrome diopside and enstatite have well-developed cleavage and parting, which unfortunately limits the size of facetable material. Enstatite produces crystals with square cross-sections when viewed down the c-axis of the crystal and diopside will have a similar cross-section with one pair of crystal surfaces that are inclined to the all other crystal faces

Chromian diopside and enstatite have comparable green color to emerald and tsavorite garnet. The primary source for chromian diopside gemstones are diamondiferous kimberlites from Siberia, where they are recovered only during warm summer months. Minor amounts are recovered by collectors and entrepreneurs in Myanmar, Pakistan, South Africa, Brazil, Italy, North America, Sri Lanka and Finland. It is a relatively inexpensive gem due to low hardness and because of rarity. Being so rare, there is little effort to market the gemstone - and with gemstones, marketing is everything. Thus the small numbers of chromian diopsides that make their way to market sell for $50 to $200/carat for faceted stones. Faceted chromian diopside gems >2 carats are rare.

Because of relatively high specific gravity, chromian diopside can be panned using a gold pan and is recovered with black sands. During the 1980s, I had a Federal grant that we used to collect >1,600 panned samples to search for hidden diamond deposits (kimberlite). About 300 'kimberlitic indicator mineral anomalies' were identified in the Laramie and Medicine Bow Mountains (Wyoming), some of which included chromian diopside. This was a 20% success rate and indicates that a major swarm of undiscovered kimberlites occurs in these areas (http://diamondprospector.webs.com). Essentially all of these remain unevaluated.

What makes chromian diopside so rare is that it is formed under great pressure and temperature within the earth’s upper mantle (similar to diamond). To get these minerals to the surface, unusual geological event(s) must have occurred. The host fractures run from the mantle at depths of 120 miles, but are only a few feet wide and at the most 1 to 2 miles long on the surface!

Age dates on the intrusives (and diamondiferous host rocks) in the Wyoming Craton tell us that kimberlite (and related magmas) erupted in multiple episodes that included: (1) prior to 2 billion years ago, (2) at the end of the Precambrian (~ 600 million years ago), (3) in Late Silurian to Early Devonian (400 to 420 million years ago), (4) the Cretaceous (140 to 70 million years ago), (5) the Tertiary (60 to 30 million years ago) and the (6) Pleistocene-Quaternary (3 to 1 million years ago).

A 2-inch long chromian diopside megacryst in kimberlite from the Sloan pipe, Colorado.

The kimberlitic magmas (as well as some lamproites and lamprophyres) acted as transporting medium for mantle rocks containing chrome diopside and individual chrome diopside megacrysts that were trapped in the magma. Such mantle rocks as pyroxenite, dunite, eclogite, lherzolite, wehrlite and harzburgite were trapped in the kimberlites – these are known as nodules or ‘xenoliths’ (foreign rock fragments).

Some of the cognate crystals and xenocrysts found in kimberlite, lamproite and lamprophyre include diamond and other rare minerals. Since these are associated with kimberlite (kimberlite is one a few magmas that originates deep enough to sample these) they have become known as ‘kimberlitic indicator minerals’ in the diamond exploration industry. The kimberlitic indicator mineral suite includes chromian diopside, chromian enstatite, pyrope garnet, picroilmenite, chromite and diamond. When kimberlites are found with sufficient quantities of diamond, it may be mined for diamond. The other gemstones, chromian diopside and pyrope garnet that could provide value-added gems to the mine, are in nearly every case ignored by mine operators due to their lower value and possibly due to ignorance. However, recovering these gems and marketing them effectively would provide added value. As it stands, only collectors who gain access to mine tailings, recover these other gemstones usually by hand sorting from the mine tailings.

Facet-quality chromian diopside found in both the State Line district (Colorado-Wyoming) and in the Green River Basin (Wyoming).

Some specimens collected in the State Line district have included chromian diopside megacrysts up to 2 inches across, impressive pyrope-almandine megacrysts up to 5 inches in diameter, and eclogite and peridotite (lherzolite) cobbles (filled with chromian diopside) up to 1.5 feet in diameter. One eclogite nodule recovered from the Sloan kimberlite contained 20% diamond! At the Iron Mountain district (Wyoming), several kimberlites have chromian diopside, but not in as great quantity as those in the State Line district.

In the southern Green River Basin, southwestern Wyoming, hundreds of anthills were identified armored with chromian diopside and enstatite, as well as beautiful, reddish-purple, reddish-pink, and yellowish-orange, transparent to translucent pyrope and almandine garnet. A few diamonds have also been found in anthills and in nearby diamondiferous lamprophyre breccias. Many of the indicator minerals are found on Cedar Mountain and in Butcherknife Draw south of Interstate 80 to the south of Fort Bridger and Green River. They have also been identified in the Bishop conglomerate along the edges of Cedar Mountain and apparently on top of Diamond Peak to the south in Colorado. This anomaly covers more than 250 square miles.

Useful maps for the State Line district include: (1) US Geological Survey topographic maps and US Bureau of Land Management Surface and Mineral Management Status maps of Ft. Collins and Laramie (1:100,000 scale); (2) General location maps of known kimberlites (see Hausel, 1998); (3) Preliminary (1:24,000 scale) geological map of the Wyoming portion of the Colorado-Wyoming State Line district (see Hausel and others, 1981). For the Green River Basin, see (1) US Geological Survey topographic maps of Firehole Canyon and Evanston (1:100,000 scale).

Elsewhere in the US, chromian diopside has been found in (1) serpentinized breccia, kimberlite and anthills at Buell Park and Garnet Ridge Arizona, (2) serpentinites in northern California; (3) kimberlites in Estes Park and the City Park of Boulder, Colorado, (4) kimberlite in Middle Sybille Creek, Wyoming, (5) the six pack lamprophyre, Wisconsin, (6) the Homestead and Williams kimberlites, Montana, (7) Green Knobs kimberlite, New Mexico; (8) Cascadilla George kimberlite, New York, (9) Winkler and several other kimberlites in northeastern Kansas, (10) kimberlites in Elliott County, Kentucky; (11) Cane Valley, Mule Ear, Mosses Rock kimberlites and serpentinized breccias, Utah; (12) Mt Horeb kimberlite, Virginia; and (13) Lake Ellen kimberlite, Michigan. And there are several other sites (see Hausel, 1998).

Several years ago, while searching for diamonds in northern California, I was searching for evidence for high pressure rocks and discovered chromian diopside in serpentinites overlying a subduction zone. In this same region, a few relatively large diamonds were found in Hayfork Creek near the Trinity River. The serpentinites were discovered near the towns of Weaverville and Hayfork.

When faceted, chromian diopside is difficult to beat. It is a beautiful stone that is as attractive if not more attractive than emerald. It is just difficult to get a consistent supply simply because no company exclusively mines for chromian diopside. It is a by-product of diamond mining and few diamond miners recover the stone as it usually reports to the mine tailings. If you watch any of the jewelry channels, you will see this stone often as supplies become available.

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Chromian Diopside - Nearly indistinguishable from Emerald and in most cases, a more attractive gemstone. Chromian diopside finds its journey in diamond pipes from Africa, Canada, Russia and Colorado. But it is also found in rare serpentinites in places like northern California.

About Me

The 'Gemhunter' is a polymath of rocks, martial arts, the stars, pencil, written word & public speaking. He is an award-winning geologist, martial artist, writer and artist. His numerous blogs on gemstones, gold and martial arts are designed to help the reader learn more about these subjects.